Various attempts have been made to provide sensing capabilities in the context of petroleum exploration, production, and monitoring, with varying degrees of success. Recently, these attempts have included the use of fiber optic cables to detect acoustic energy. Because the cables typically comprise optically conducting fiber containing a plurality of backscattering inhomogeneities along the length of the fiber, such systems allow the distributed measurement of optical path length changes along an optical fiber by measuring backscattered light from a laser pulse input into the fiber. Because they allow distributed sensing, such systems may be referred to as “distributed acoustic sensing” or “DAS” systems. One use of DAS systems is in seismic applications, in which seismic sources at known locations transmit acoustic signals into the formation, and/or passive seismic sources emit acoustic energy. The signals are received at seismic sensors after passing through and/or reflecting through the formation. The received signals can be processed to give information about the formation through which they passed. This technology can be used to record a variety of seismic information. Another application is in the field of in-well applications and acoustic fluid monitoring.
DAS systems typically detect backscattering of short (1-10 meter) laser pulses from impurities or inhomogeneities in the optical fiber. If fiber is deformed by an incident seismic wave then 1) the distance between impurities changes and 2) the speed of the laser pulses changes. Both of these effects influence the backscattering process. By observing changes in the backscattered signal it is possible to reconstruct the seismic wave amplitude. The first of the above effects appears only if the fiber is stretched or compressed axially. The second effect is present in case of axial as well as radial fiber deformations. The second effect is, however, several times weaker than the first. Moreover, radial deformations of the fiber are significantly damped by materials surrounding the fiber. As a result, a conventional DAS system with a straight fiber is mainly sensitive to seismic waves polarized along the cable axis, such as compression (P) waves propagating along the cable or shear (S) waves propagating perpendicular to the cable. The strength of the signal varies approximately as cos2 θ, where θ is the angle between the fiber axis and the direction of wave propagation (for P waves). Thus, while there exists a variety of commercially available DAS systems that have varying sensitivity, dynamic range, spatial resolution, linearity, etc., all of these systems are primarily sensitive to axial strain. Acoustic signals travelling normal to the fiber axis may sometimes be referred to as “broadside” signals and, for P waves, result in radial strain on the fiber. Thus, as the angle between direction of travel of the acoustic signal and the fiber axis approaches 90°, DAS cables become much less sensitive to the signal and may even fail to detect it. The Appendix attached hereto provides further discussion of the mathematics of sinusoidal fibers.
Hence, it is desirable to provide an improved cable that is more sensitive to signals travelling normal to its axis and enables distinguishing radial strain from the axial strain. Sensitivity to broadside waves is particularly important for seismic or microseismic applications, with cables on the surface or downhole. In addition to broadside sensitivity, it is also desirable to provide three-component (3C) sensing, from which the direction of travel of the sensed signal can be determined.